Origin of High Helium Concentrations in Dry Gas by Water Fractionation
Alton A. Brown
Consultant, Richardson, Texas, [email protected]
A small but significant fraction of natural gas deposits have helium concentrations much greater than those in water and rock. The undisputed abiogenic origin of helium has been used as evidence for basement origin for these gases and for methane (Gold and Held 1987). However, models of helium fraction between migrating gas and stationary water demonstrate that natural gas migrating through water-saturated rock can develop high helium concentration regardless of the methane source. For this reason, the presence of inorganic gases in hydrocarbon gas deposits cannot be used as evidence for the origin of the hydrocarbons in the gas. These concepts are applied to the gases of the Hugoton-Panhandle field area, recently discussed by Ballentine and Sherwood-Lollar (2002).
The first control for He concentration in a gas is Helium (He) concentration in water immediately before gas migration. He rarely has high enough concentration to form a gas phase in buried water-saturated sedimentary rocks, so He accumulates in pore water until a gas phase interacts with the water. As gas enters the porous rock, essentially all He partitions from water into the gas phase, because the Henry's constant for He is very high (model 1 of Ballentine and Sherwood-Lollar (2002). The maximum possible He fraction in the gas is approximately equivalent to the helium saturation in pore water prior to migration. Older, deeper reservoirs are likely to accumulate more He by in situ generation from uranium fission and diffusion from underlying basement than young reservoirs. High He concentration is therefore more likely in gases that have migrated through older reservoirs. The second control on He concentration is the methane/water ratio. As gas migrates through the reservoir, it removes most He from pore water. Subsequent methane moving on the same pathway acquires less He, because prior migration has removed essentially all He. The first gas to reach the trap will have the highest He concentration. As more gas is delivered along the same migration pathway, the concentration of gas is diluted by the later, He-poor gas. In most settings, this dilutes He in large methane accumulations.
The largest He accumulations result from basin exhumation. As pressure decreases during exhumation, gases exsolve from water at all positions and depths in the basin where the sum of the dissolved gases partial pressure is near to the total pressure. If a gas phase forms, most trace He and nitrogen fractionate into the gas phase. Because all gas has relatively high He concentrations, large accumulations are not diluted by He-poor gases migrating on the same pathway.
Most gas phases in sedimentary basins are hydrocarbons, so the association between He, other inert gases such as nitrogen, and methane is inevitable, regardless of the origin of the methane. Spatially variable He concentration and locally consistent He/N2 ratios are direct consequences of this process. He/N2 ratio is controlled by initial saturation ratio in water and ratio of Henry's constants at dissociation conditions. This model also explains (1) why high He gases are rare, (2) the well known association between high helium concentration and old sedimentary reservoirs (Katz 1969) and (3) the inverse relation between concentration and total size of high-helium deposits (Nikonov 1972). These are characteristics unexpected with a basement origin for both methane and He.
Ballentine, C. and B. Sherwood-Lollar, 2002, Regional groundwater focusing of nitrogen and noble gases into the Hugoton-Panhandle giant gas field: Geochimica et Cosmochimica Acta, v. 66, p. 2483-2497.
Gold, T. and M. Held, 1987, Helium-nitrogen-methane systematics in natural gases of Texas and Kansas: Journal of Petroleum Geology, v. 10, p. 415-424.
Katz, D. L., 1969, Source of helium in natural gas: United States Bureau of Mines information circular 8417, p. 242-255.
Nikonov, V. F., 1973, Formation of helium-bearing gases and trends in prospecting for them: International Geology Review, v. 15 #5, p. 534-541 (translated from Sovetskaya Geologiya, 1972, #7, p. 105-115.).